This invention relates to a drum brake device; more specifically, it relates to a drum brake device that functions as a leading-trailing (LT) type when the service brake is applied, and as a duo-servo (DS) type when the parking brake is applied.
This type of drum brake device has been disclosed in Australian patent number AU-B1-53 491/79 and U.S. Pat. No. 5,275,260. The basic brake functions are the same in both devices, and the structure will be explained with reference to FIG. 15. A pair of brake shoes b, c are provided on top of a back plate a. An anchor block d is provided between the one adjacent ends of each brake shoe, and a hydraulic cylinder g is provided between the other adjacent ends of each brake shoe. A parking lever j is pivoted on one end i of one brake shoe b, and an idler lever k is pivoted so as to swing on the other brake shoe c. The first and second rods l, m are provided between the two brake shoes b, c, wherein one end n of the first rod l engages with the parking lever j, and the other end o engages with the idler lever k; and one end p of the second rod m engages with one brake shoe b, while the other end q engages with the other brake shoe c and the idler lever k.
The brake action is explained as follows. When the driver steps on the brake pedal, the hydraulic cylinder g is pressurized, wherein the two brake shoes b, c spread open, with the point of abutment with the anchor block d as the fulcrum, to cause a frictional connection with the rotating brake drum, not shown in the diagram, in a leading-trailing braking action.
When the parking brake is applied, the parking lever j is pulled in the direction of the arrow X. The force of that action is transferred in sequence to the first rod l, idler lever k, and second rod m, wherein the one brake shoe b opens with the point of abutment with the anchor block d as the fulcrum to cause a frictional connection with the brake drum. Next, the idler lever k turns, with the point of abutment with the second rod m as the fulcrum, causing the pivot component of the idler lever k to press the other brake shoe c in the direction of the arrow Y to cause a frictional connection with the brake drum. At the same time, the reaction force of the parking lever j is being applied in the direction of the arrow Z on the one end i of the one brake shoe b.
If at this time, the torque is applied on the brake drum in the direction of arrow R (uphill or downhill), the friction force of the one brake shoe b is transferred to the second rod m, wherein its other end q presses against the other brake shoe c, supported by the anchor block d, in a duo-servo braking action. If the torque is applied on the brake drum in the opposite direction of arrow R, the friction force of the other brake shoe c is transferred to the second rod m, wherein its one end p is pressed against the one brake shoe b, supported by the anchor block d, in the same duo-servo braking action as above.
As is evident from this parking brake operation, if the other end q of the second rod m abuts the idler lever k and a gap exists between the other end q and the other brake shoe c, then when the shoe c rotates in the opposite direction of R, the piston of the hydraulic cylinder g is repelled to an amount equivalent to the gap. That is, the brake pedal is repelled which is not only disconcerting to the driver, but the pedal stroke increases in the next brake pedal application. Conversely, if the other end q of the second rod m abuts against the other brake shoe c and a gap exists between the other end q and the idler lever k, then the stroke of the parking lever j increases by an amount equivalent to this gap; that is, the stroke of the hand lever increases. From these perspectives, it is preferable that the gap between the other end q of the second rod m and either the other brake shoe c or the idler lever k be as small as possible.
Next, FIG. 16 illustrates the concept of the automatic shoe clearance device which is installed in the drum brake device of U.S. Pat. No. 5,275,260. The bent end y of the adjustment lever r is pivotable on the web of the brake shoe c, and one end of the upper arm s is connected to the groove of the upper strut t for their interaction. Another arm is connected to the star wheel u of the upper strut t. A spring w, stretched between the adjustment lever r and the pivot lever v, energizes the adjustment lever r in the counterclockwise direction, with the end y as the fulcrum.
Should the brake lining be worn causing the two brake shoes b, c to open by more than a prescribed value when the service brake is applied, the upper arm causes the star wheel u to rotate to automatically extend the entire length of the upper strut t, thereby maintaining a constant clearance between the brake shoes b, c and the brake drum z.
Problems to be resolved by this invention
The drum brake device as described above has need of improvement in the following areas:
When the idler lever k is pivoted to rotate on the other brake shoe c, it is very difficult to check the proper alignment of the components.
Applying the parking brake and the brake drum which is engaged with the wheels starts to rotate with the device in this state, both brake shoes, the rods, or struts and other components all turn in tandem, wherein one or the other brake shoe collides against the anchor block. The noise so generated is disconcerting to the driver. Additionally, this impact load applied repeatedly on the anchor block can diminish the structural strength of the components. Therefore, the strong but heavier materials were used in the conventional brakes.
In the conventional device as disclosed in the Australian Patent Number AU-B1-53 491/79, the cumulative effect of the tolerances of each component will be such that the idler lever could abut against the second rod or play could be generated. At the very least, gap will be generated when the service brake is applied. Accordingly, the idler lever could vibrate creating a strange noise when the vehicle is in motion or the service brake is applied. Again, this noise can be disconcerting to the driver.
Moreover, as the lining of the other brake shoe c gradually wears, there is a gradual shift in the point at which the second rod m contacts with the brake shoe c or the idler lever k. That is, as shown in FIG. 15, the amount of displacement .delta. of the brake shoe c at the pivot point of the brake shoe with the link, and the amount of displacement .delta.c and .delta.k of the brake shoe c and idler lever k respectively at the point of the contact with the second rod m are defined as follows: ##EQU1## H1: Distance from anchor d to the brake center (pivot point of brake shoe c and idler lever k);
H2: Distance from the pivot point of the brake shoe and the idler lever k to the second rod m; PA0 H3: Distance from the pivot point of the brake shoe and the idler lever k to the first rod l; PA0 .delta.: Amount of lining wear (amount of displacement).
In this case, H3 is considerably smaller than H1, hence the displacement .delta.k of the idler lever k will be considerably larger than the displacement .delta.c of the brake shoe c. As a result, in the conventional device as disclosed in U.S. Pat. No. 5,275,260, as the lining wears, the adjustment lever becomes interlocked with the brake shoe and moves with its point of abutment with the support block as the fulcrum, while the pivot lever moves with its point of abutment with the lower strut as the fulcrum. This changes the energizing force of the adjustment spring energizing the adjustment lever, which has a negative effect on the automatic adjustment process when only a very minimal adjustment is required.
Moreover, in the conventional drum brake device, there are chances that the brake shoe bites the brake drum causing unusual dragging between the brake lining and the brake drum. As a result, the wheels may be locked creating a dangerous situation to the driver.